Fossile fuel combustors discharge flue-gases at elevated temperatures (the flue-gases from steam raising boiler plant for example, are normally discharged at a temperature somewhat above the temperature of the steam produced in the boiler) usually via a tall chimney to the atmosphere.
This method of operating produces an air draft which disperses noxious gases and alleviates acid condensate and its corrosive effects. The method does not however prevent atmospheric pollution; and the discharge of hot gases to the atmosphere means that there is a considerable thermal loss from the combustor with the result that the overall operating efficiency of the combustor is lower than would otherwise be the case.